Opioid analgesics are the most efficient tool in pain management; however, high tolerance and addiction liabilities also make them the most abused class of medication, resulting in millions of cases of drug overdose and more than 15,000 deaths in the US each year. Specific modulation of ?-opioid receptor (DOR) represents one of the most promising strategies to control tolerance and addiction to opioid drugs; however, only a few chemotypes of DOR-selective compounds have been explored. A new unique opportunity to expand the repertoire of DOR compounds by discovery of novel chemotypes with desirable functional profiles is enabled by structural characterization of opioid receptors, including new high resolution structures of DOR/naltrindole complex at 1.8 , and DOR with bifunctional peptide DIPP-NH2. The structures reveal atomic details of ligand binding and provide a reliable 3D framework for rational discovery of new ligands targeting orthosteric or allosteric pockets of DOR. The project will primarily focus on two therapeutically beneficial profiles: (a) bifunctional MOR agonists/DOR antagonists known to reduce tolerance and side effects of opioid therapy, and (b) allosteric ligands that modulate DOR activity only in the presence of endogenous opioid. In the exploratory R21 phase of the project we will perform large scale structure-based virtual ligand screening of more than 6 million available compounds to select small sets of candidate hits, and then experimentally test and identify new bifunctional and allosteric ligands. The most promising chemical scaffolds, and validated structural models will be employed then in an iterative lead optimization at the R33 phase. Both initial screening and lead optimization phases will employ state-of-the-art structure-based approaches which have proven to be highly effective in recent applications to GPCRs. Success of the program will result in new insights into drug action, lead to new DOR ligands with desired properties, and establish a solid platform for rational structure-based discovery of safer and less addictive opioid therapies.